Prior art, dual-delayed sweep oscilloscopes have two positionable markers on a display. Each marker may be positioned to coincide with the display of one of the two recurring events. The markers are selectively positioned by adjusting first and second controls which vary the levels of first and second reference voltages, respectively. A comparator compares one of the variable reference voltages to a horizontal sweep signal during alternate horizontal sweep signals and when the level of the horizontal sweep signal exceeds the level of the variable reference voltage, the intensity of the display is increased to define the positionable marker. Since the level of the horizontal sweep signal increases linearly with time, the time interval between the initiation of the horizontal sweep signal and each recurring event can be determined by dividing the level of each reference voltage by the slope of the horizontal sweep signal. The time interval between the two recurring events is determined by dividing the difference between the levels of the two reference voltages by the slope of the horizontal sweep signal.
The accuracy of the prior art oscilloscopes is limited by the accuracy of the calibration of the slope of the horizontal sweep signal, non-linearities in the horizontal sweep signal, and by errors in measuring the difference between the levels of the two reference voltages.
Further, due to noise in the circuitry which generates the horizontal sweep signal, the comparator is susceptible to triggering on the noise instead of the horizontal sweep signal when a low level reference voltage is selected. Unpredictable triggering of the comparator as a result of this noise susceptibility causes instability of the marker position and errors in the time measurement. Further still, the leading portion of the horizontal sweep signal is typically non-linear which causes additional errors in measuring time intervals associated with low level reference voltages. To avoid these problems, prior art oscilloscopes typically establish a minimum level for the variable reference voltages. However, the use of a minimum level for the variable reference voltages results in a time interval subsequent to the initiation of the horizontal sweep signal in which positionable markers cannot be positioned. Therefore, prior art oscilloscopes cannot make time measurements in this time interval.
Finally, the prior art does not provide any method for measurement of the time interval between two triggered events. The prior art merely teaches a method for manually positioning intensified markers to coincide with desired events and for determining the time interval between the manually positioned markers.
A typical prior art oscilloscope is described in detail in U.S. Pat. No. 3,975,684, entitled SWEEP GENERATOR APPARATUS AND METHOD issued to William J. Mordan on Aug. 17, 1976.